For an object with negative mass, Isaac Newton’s second law of motion, in which a force is equal to the mass of an object multiplied by its acceleration (F=ma) would be experienced in reverse.
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Scientists have created a fluid that exhibits the bizarre property of “negative mass” in an experiment that appears to defy the everyday laws of motion.

Push an object and Newton’s laws (and common experience) dictate that it will accelerate in the direction in which it was shoved.

“That’s what most things that we’re used to do,” said Michael Forbes, a physicist at Washington State University and co-author of the paper, which shows that normal intuitions do not always apply to physics experiments. “With negative mass, if you push something, it accelerates toward you.”

Negative mass has previously cropped up in speculative theories, including those suggesting the existence of wormholes, a form of cosmological shortcut between two points in the universe. Just as electric charge can be either positive or negative, matter could, hypothetically, have either positive or negative mass.

For an object with negative mass, Newton’s second law of motion, in which a force is equal to the mass of an object multiplied by its acceleration (F=ma) would be experienced in reverse.

Theoretically, this sounds straightforward, but picturing how this behaviour would work in the real world is bewildering, even for experts.

“It’s very counterintuitive and weird,” said Jon Butterworth, a physicist at University College London, who was not involved in the latest work.

For instance, you might expect a ball with negative mass to be repelled from the Earth’s surface, but theory predicts that it would behave just like ordinary matter and fall downwards.

No fundamental particles with negative mass have ever been discovered, meaning that there have never been any experimental insights into how they might behave – if, indeed, they exist. The latest study provides a new platform to study this hypothetical form of matter, by showing that under certain precise conditions, normal particles can be made to behave as though they had negative mass.

“It provides another environment to study a fundamental phenomenon that is very peculiar,” said Forbes.

The experiment, described in the journal Physical Review Letters, created the conditions for negative mass by cooling rubidium atoms to just above absolute zero, creating something called a Bose-Einstein condensate.

In this cooled state, particles move extremely slowly and, following the principles of quantum mechanics, behave like waves. This state of matter is already known to exhibit strange properties such as superfluidity, in which a liquid can creep up the sides of jars and over the top.

The cooling was achieved by using lasers to slow the particles until they were confined in a laser trap, less than 100 microns across. Breaking the trap causes the rubidium atoms to rush out, expanding in a spherical formation.

However, when researchers applied a second set of lasers that kicked the atoms back and forth within the trap, they started to behave as though they had negative mass on exiting the trap.

“Once you push, it accelerates backwards,” said Forbes, who acted as a theorist analysing the system. “It looks like the rubidium hits an invisible wall.”

Martin McCall, a professor of theoretical optics at Imperial College London, described the paper as a neat demonstration of a system exhibiting “effective negative mass” – something that has only rarely been created before in laboratory conditions.

But does it tell us anything about the possibility of cosmological wormholes or how hypothetical exotic particles might behave? “Personally I doubt it,” said McCall.